An equatorium (plural, equatoria) is an astronomical calculating instrument. It can be used for finding the positions of the Moon, Sun, and planets without calculation, using a geometrical model to represent the position of a given celestial body.
The earliest extant record of a solar equatorium, that is, one to find the position of the sun, is found in Proclus's fifth-century work Hypostasis, where he gives instructions on how to construct one in wood or bronze. Although planetary equatoria were also probably made by the ancient Greeks, the first surviving description of one is from the Libros del saber de astronomia (Books of the knowledge of astronomy), a Castilian compilation of astronomical works collected under the patronage of Alfonso X of Castile in the thirteenth century, which includes translations of two eleventh century Arabic texts on equatoria by Ibn al‐Samḥ and al-Zarqālī. Theorica Planetarum (c. 1261-1264) by Campanus of Novara describes the construction of an equatorium, the earliest known description in Latin Europe.
Richard of Wallingford (1292–1336) is known to have built a sophisticated equatorium named Albion in 1326. It could calculate lunar, solar and planetary longitudes. Unlike most equatoria, the Albion could also predict eclipses. The device is described in a manuscript and in drawings by the Abbot. It consisted of several rotating disks, showing the courses of the sun, moon and stars. These disks were operated manually. It was not a clockwork mechanism.
- Proclus (1909). Hypotyposis Astronomicarum Positionum. Bibliotheca scriptorum Graecorum et Romanorum Teubneriana. Karl Manitius (ed.). Leipzig: Teubner.
- Evans, James (1998). The History and Practice of Ancient Astronomy. Oxford & New York: Oxford University Press. p. 404. ISBN 978-0-19-509539-5.
- Toomer, G. J. (1971). "Campanus of Novara". In Gillispie, Charles Coulston. Dictionary of scientific biography III. New York: Scribner. pp. 23–29. ISBN 978-0-684-10114-9
- Morrison, James E. "Richard of Wallingford". History of Astronomy.
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